The present invention relates to an analog/digital converter with a dither signal superposed at the input side. Prior art converters are found in the following references: Manfred Bartz, Large-Scale Dithering Enhances ADC Dynamic Range, Microwaves&RF, May 1993; Large-Scale-Dithered-ADC, Hewlett-Packard Journal, December 1993; Wagdy M.F., Linearizing Average Transfer Characteristics of Ideal ADCs via Analog and Digital Dither, IEEE Transactions on Instrumentation and Measurement, vol.43, no. 2, April 1994; DE 36 38 006; EP 0613 256; WO 87/06079. By means of the dither signal, both the differential and the integral linearity errors of the A/D converter can be improved.
The known use of a noise signal or, respectively, pseudo-noise signal as a dither signal (International reference WO 87/06079) has the disadvantage that the noise signal has a high crest factor (ratio of the peak to the RMS value), which, already at a relatively low RMS value, leads to a significant adverse effect on the useful dynamic range that is the output range. In practice, a crest factor of 5 to 6 can be assumed for a noise signal, which means that the RMS value of the dither signal must not be greater than 4 to 5% of the output range of the A/D converter.
It is also known to use a pure sine signal as a dither signal (German reference DE 36 38 006). By means of such a sine signal, a broad line spectrum with strongly pronounced individual secondary discrete spectral components is produced at the integral non-linearity of the A/D converter, which is disturbing in certain applications, e.g. in a spectrum analyzer.
Corresponding considerations hold for another known measure, namely the use of a pulse-width-modulated periodic triangular or rectangular signal as a dither signal (European reference EP 0 613 256). This known measure, provided for increasing resolution of an A/D converter, leads again spectrally to preferred discrete spectral components, which are disturbing in various applications of such A/D converters, e.g. in spectrum analysis or in digital receivers with IF sampling.
These known modulated or unmodulated sine signals are thus also not suited as dither signals for all applications.